Silica with a chemical name of hydrated amorphous silica or colloidal silica (SiO2.nH2O) is a white, non-toxic and amorphous fine powder. SiO2 content in silica is greater (>90%), primary particle size is generally 10-40 nm and aggregation of the particles is due to more hydrophilic hydroxy groups on the surface, which can absorb water easily. The relative density of silica is 2.319˜2.653 t/m3 and melting point is 1750° C. It is insoluble in water and acid but soluble in alkali and hydrofluoric acid. Moreover, silica has the advantages of good porosity, large inner surface area, high dispersibility, light weight, good chemical stability, resistance to high temperature, resistance to combustion, good electrical insulation, and so on. Further, silica has a crosslinked structure randomly connected by a covalent bond of —O—Si—O—, the structural unit of which may be aggregated into a once, twice or even three times agglomerate. Silica is mainly used as a kind of filler for rubbers, plastics, synthetic resins and paints and also can be used as a kind of lubricant and an insulating material. Currently, 70% of silica in the world is used as a kind of good reinforcing agent in rubber industry for improving bonding performance and tear resistance.
Silica is hydrophilic owing to internal silicone, active silanol groups on outer surface and the adsorbed water; so, it is difficult to be wet and dispersed in an organic phase; moreover, the agglomerates are always inclined to agglomerate because of greater surface energy coming from the hydroxy groups on the surface. So, application performances of the products are affected, e.g. in the rubber curing system, unmodified silica can not be well dispersed in the polymer so that coupling bonds among fillers and the polymers are difficult to form, which will reduce curing efficiency and reinforcement properties. Surface modification of silica is to eliminate or reduce the amount of the active silanol groups through reacting the hydroxy groups with a certain chemical substances by a certain process, which can make the products change from hydrophilicity to hydrophobicity as well as increase its dispersibility in the polymers.
Currently, the modification methods of silica include dry modification, wet modification and hot-pressing modification. The wet modification has the advantages of small amount of a modifying agent and low cost, however, large amounts of organic solvents used make post-processing complicated and could easily lead to environmental pollution. The dry modification avoids the disadvantages of the wet modification with a simple process, a large processing capacity, a simple post-processing, high reaction efficiency, but still has the disadvantages of stringent operating conditions, large amounts of the modifying agent, high modification cost and nonuniform modification. Moreover, the hot-pressing modification has the drawbacks of high energy consumption, high equipment requirements, relatively small processing load, large amounts of the modifying agent and nonuniform modification.